1. Field of the invention:
The present invention relates to a field effect transistor-type sensor for detecting a variation of the gate operation of a field effect transistor due to a variation of external factors, by a sensitive means formed on the gate insulating film of an MOS- or MIS-field effect transistor.
2. Description of the prior art:
A field effect transistor (hereinafter, referred to as FET)-type sensor, which comprises an FET device incorporated with a sensitive means exhibiting an electric variation of electrostatic capacity, electric conductivity, electrostatic potential, etc., due to a physical or chemical interaction with the physical quantity to be detected, detects the said physical quantity as a variation of the gate operation of the said FET device. Taking advantage of the high input impedance and the amplifying function of the FET device, such an FET type sensor can exhibit a high output, even though its size is extremely small, and thus is advantageous in actual use. Particularly, an FET type sensor which is constructed in such a manner to have a sensitive means on the gate region of the FET device is advantageous practically and economically since the FET device can be small, and a number of devices can be formed on the same substrate. However, such an FET type sensor containing the FET device therein is inferior to an ordinary FET device alone in the operation stability of the FET device. It is also inferior to an FET device with regard to the output stability and the reproducibility of the output characteristic. Depending upon the kind of the FET type sensor required, materials and production processes of the sensitive means are so different that the operation characteristic of the FET device can be remarkably varied. As compared with an ordinary FET device, a large amount of impurities and/or ions are apt to appear in the sensitive means or may contaminate the interface between the sensitive means and the gate insulating film during the formation of the sensitive means on the FET device, causing instability not only in the operation characteristic of the FET device but also in the output characteristic of the FET type sensor. Moreover, since the FET type sensor, which is designed to be used as an atmosphere sensor such as a gas sensor, a moisture sensor, etc., is exposed to an atmosphere, it will be contaminated by impurities in the atmosphere, causing variation and/or deterioration of the FET characteristic and/or deterioration of the sensor itself. Accordingly, an FET type sensor must be provide with an arrangement which will suppress the influence of impurities and/or ions contained in the materials of the sensitive means, or impurities and/or ions contaminating the interface between the sensitive means and the gate insulating film during the formation of the sensitive means on the FET device and/or during operation of the FET device, thereby providing for a stable output characteristic over a long period of time. If such an FET type sensor is designed, a variety of sensors such as gas sensors, moisture sensors, ion sensors, biological sensors, infrared-ray sensors, etc. will be able to be produced in an FET type format. FET type gas sensors, moisture sensors, ion sensors and biological sensors cannot avoid direct interaction of the sensitive means with the atmosphere so that the device therein cannot be covered with a package, etc. Therefore, the above-mentioned problems deriving from the contamination etc. of impurities and/or ions from the outside must be solved for FET type sensors. To solve these problems, a silicon nitride film having a small diffusion coefficient for ions, moisture, etc. has been used as the gate insulating film, or used to cover the surface of the FET device. The resulting FET sensors are, however, still inferior in their output stability over a long period of time.